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United States Patent |
5,631,096
|
Nakajima
,   et al.
|
May 20, 1997
|
Magneto optical memory device
Abstract
A magneto-optical memory device is provided with a base whereon a first
magnetic film which exhibits in-plane magnetization at room temperature
and exhibits perpendicular magnetization at above room temperature, a
second magnetic film having its Curie temperature above room temperature;
and a third magnetic film having its Curie temperature set above the Curie
temperature of the second magnetic film, which exhibits perpendicular
magnetization in a temperature range between room temperature and Curie
temperature are laminated in this order. When recording, the temperature
of the third magnetic film is raised to the vicinity of its Curie
temperature, and information is recorded thereon by an external magnetic
field. As the magnetization of the second magnetic film having a
temperature rise above its Curie temperature disappears, an exchange
coupling force is not exerted between the first magnetic film and the
third magnetic film. In the above arrangement, since the effect from the
magnetization of the first magnetic film can be avoided, information can
be recorded on the third magnetic film by a small external magnetic field,
thereby permitting a reduction in electric power consumption and in the
size of the apparatus.
Inventors:
|
Nakajima; Junsaku (Yamatotakada, JP);
Murakami; Yoshiteru (Nishinomiya, JP);
Ohta; Kenji (Kitakatsuragi-gun, JP);
Takahashi; Akira (Nara, JP)
|
Assignee:
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Sharp Kabushiki Kaisha (Osaka, JP)
|
Appl. No.:
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400464 |
Filed:
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March 7, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
428/819; 365/122; 369/13.4; 369/13.42; 428/900 |
Intern'l Class: |
G11B 005/66 |
Field of Search: |
428/694 ML,694 SC,694 RE,694 NF,900,694 MM,694 EC
369/13
365/122
|
References Cited
U.S. Patent Documents
5278810 | Jan., 1994 | Takahashi et al. | 369/13.
|
Foreign Patent Documents |
258978 | Mar., 1988 | EP.
| |
0318925A3 | Jun., 1989 | EP.
| |
0352548A2 | Jan., 1990 | EP.
| |
0498459A2 | Aug., 1992 | EP.
| |
0509836A2 | Oct., 1992 | EP.
| |
Other References
M. Kaneko et al., Jpn. J. Appl. Phys., 31, Pt. 1, No. 2B, pp. 568-575
((1992).
|
Primary Examiner: Kiliman; Leszek
Attorney, Agent or Firm: Conlin; David G., Oliver; Milton
Parent Case Text
This is a continuation of application Ser. No. 08/102,553, flied on Aug. 5,
1993, now abandoned.
Claims
What is claimed is:
1. A magneto-optical memory medium, comprising:
a first magnetic film which exhibits in-plane magnetization at room
temperature and exhibits perpendicular magnetization in a predetermined
temperature range above room temperature;
a second magnetic film whose Curie temperature (T2) is set above room
temperature; and
a third magnetic film which exhibits perpendicular magnetization in a
temperature range between room temperature and its Curie temperature (T3),
wherein said third magnetic film is an alloy whereon recorded information
is rewritten by overwriting using a magnetic field intensity modulation,
said first magnetic film, said second magnetic film and said third magnetic
film being laminated in this order on a base substrate,
wherein:
the Curie temperature (T2) of said second magnetic film is set lower than
the Curie temperature (T3) of said third magnetic film;
in a temperature range from not less than room temperature to less than
Curie temperature (T.sub.3) of said third magnetic layer, said third
magnetic layer has a coercive force H.sub.c3 which is always stronger than
a coercive force H.sub.c2 of said second magnetic layer; and
the Curie temperature (T3) of said third magnetic film is lower than the
Curie temperature (T1) of said first magnetic film.
2. The magneto-optical memory medium as set forth in claim 1, wherein:
said third magnetic film is a recording film for recording thereon
information based on a magnetization direction.
3. The magneto-optical memory medium as set forth in claim 2, wherein:
said first magnetic film is a read-out film whereon a light beam is
projected from the side of said base so as to raise the temperature of the
portion thereof corresponding to a central portion of the light beam in a
predetermined temperature range;
as the temperature rises, a transition occurs in said first magnetic film
from in-plane magnetization to perpendicular magnetization;
the information recorded on said third magnetic film is copied through said
second magnetic film to said first magnetic film; and
by projecting the light beam, the copied information is read out utilizing
the Kerr effect.
4. The magneto-optical memory medium as set forth in claim 2, wherein:
when the temperature of said third magnetic film is raised to the vicinity
of its Curie temperature (T3) by projecting a light beam, so as to record
thereon information by applying thereto an external magnetic field, the
magnetization of said second magnetic film disappears, and said second
magnetic film serves as a switching film by cutting off influence of the
magnetization of said first magnetic film on the magnetization of said
third magnetic film.
5. The magneto-optical memory medium as set forth in claim 1, wherein:
said first magnetic film is made of a rare-earth/transition metal alloy
whose composition is set at rare-earth-moment-rich composition at room
temperature, indicating that the magnetic moment of said rare-earth metal
is greater than that of said transition metal at room temperature.
6. The magneto-optical memory medium as set forth in claim 5, wherein:
said first magnetic film exhibits in-plane magnetization in a temperature
range between room temperature and a temperature below 130.degree. C.,
whereas, it exhibits perpendicular magnetization in a temperature range of
130.degree.-280.degree. C.
7. The magneto-optical memory medium as set forth in claim 6, wherein:
said first magnetic film is made of Gd.sub.0.26 (Fe.sub.0.80
Co.sub.0.20).sub.0.74.
8. The magneto-optical memory medium as set forth in claim 1, wherein:
said first magnetic film is made of rare-earth transition metal alloy, and
it exhibits in-plane magnetization in a temperature range between room
temperature and a temperature below 70.degree. C., whereas, it exhibits
perpendicular magnetization in a temperature range of 70.degree. C.--Curie
temperature.
9. The magneto-optical memory medium as set forth in claim 8, wherein:
said first magnetic film is made of Gd.sub.0.28 (Fe.sub.0.8
Co.sub.0.2).sub.0.72.
10. The magneto-optical memory medium as set forth in claim 8, wherein:
said first magnetic film is made of Gd.sub.0.28 (Fe.sub.0.82
Co.sub.0.18).sub.0.72.
11. The magneto-optical memory medium as set forth in claim 1, wherein:
said first magnetic film is made of GdCo.
12. The magneto-optical memory medium as set forth in claim 11, wherein:
said first magnetic film is made of Gd.sub.0.25 Co.sub.0.75.
13. The magneto-optical memory medium as set forth in claim 1, wherein:
said first magnetic film is made of Dy.sub.0.3 (Fe.sub.0.7
Co.sub.0.3).sub.0.7.
14. The magneto-optical memory medium as set forth in claim 1, wherein:
said second magnetic film is rare-earth-moment-rich in a temperature range
between room temperature and Curie temperature;
said second magnetic film exhibits in-plane magnetization in the
temperature range between room temperature and Curie temperature; and
said second magnetic film is made of a rare-earth/transition metal alloy
whose composition is set such that its Curie temperature is set higher
than the lowest temperature at which said first magnetic film exhibits
perpendicular magnetization, rare-earth-moment-rich in a temperature range
between room temperature and Curie temperature, indicating that the
magnetic moment of said rare-earth metal is greater than that of said
transition metal in a temperature range between room temperature and Curie
temperature.
15. The magneto-optical memory medium as set forth in claim 14, wherein:
said second magnetic film is made of GdFeCo.
16. The magneto-optical memory medium as set forth in claim 15, wherein:
said second magnetic film is made of Gd.sub.0.28 (Fe.sub.0.90
Co.sub.0.10).sub.0.72.
17. The magneto-optical memory medium as set forth in claim 1, wherein:
said second magnetic film is transition-metal-moment-rich in a temperature
range between room temperature and Curie temperature;
said second magnetic film exhibits in-plane magnetization in the
temperature range between room temperature and Curie temperature; and
said second magnetic film is made of rare-earth/transition metal alloy
whose composition is set such that its Curie temperature is set higher
than the lowest temperature at which said first magnetic film exhibits
perpendicular magnetization, is transition-metal-moment-rich in a
temperature range between room temperature and Curie temperature,
indicating that the magnetic moment of said transition metal is greater
than that of said rare-earth metal, in a temperature range between room
temperature and Curie temperature.
18. The magneto-optical memory medium as set forth in claim 17, wherein:
said second magnetic film is made of DyFeCo.
19. The magneto-optical memory medium as set forth in claim 18, wherein:
said second magnetic film is made of Dy.sub.0.22 (Fe.sub.0.90
Co.sub.0.10).sub.0.78.
20. The magneto-optical memory medium as set forth in claim 1, wherein:
said third magnetic film is made of rare-earth/transition metal alloy whose
composition is set such that it is transition-metal-moment-rich in a
temperature range between room temperature and its Curie temperature.
21. The magneto-optical memory medium as set forth in claim 20, wherein:
said third magnetic film is made of Dy.sub.0.23 (Fe.sub.0.80
Co.sub.0.20).sub.0.77.
22. The magneto-optical memory medium as set forth in claim 1, wherein:
said third magnetic film is made of Dy.sub.0.23 (Fe.sub.0.82
Co.sub.0.18).sub.0.77.
23. The magneto-optical memory medium as set forth in claim 1, wherein:
a first dielectric film having a characteristic that light can be
transmitted therethrough is formed between said base and said first
magnetic film, said first dielectric film serving as a protective coat;
a second dielectric film having a characteristic that light can be
transmitted therethrough is formed on said third magnetic film, said
second dielectric film serving as a protective coat; and
a reflecting film is formed on said second dielectric film for enhancing
the magneto-optical effect.
24. The magneto-optical memory medium as set forth in claim 23, wherein:
said first and second dielectric films are made of AlN;
said reflecting film is made of Al; and
said base is composed of a base having a characteristic that light can be
transmitted therethrough.
Description
FIELD OF THE INVENTION
The present invention relates to a magneto-optical memory device such as a
magneto-optical disk, etc., on or from which recording, erasing, or
reproducing of information is permitted using light such as a laser beam.
BACKGROUND OF THE INVENTION
As an example of a magneto-optical memory device, a magneto-optical disk
provided with a substrate whereon a first dielectric film, a
recording-reproduction film, a second dielectric film, a reflecting film,
and an overcoat film are laminated in this order is known.
For the recording-reproduction film, a thin film of rare-earth transition
metal alloy (RE-TM) having perpendicular magnetic anisotropy such as
DyFeCo, TbFeCo, or GdTbFe is used.
When recording is to be carried out on the magneto-optical disk, light such
as a laser beam is projected onto the recording-reproduction film. As a
result, temperature of the portion irradiated with the light is raised,
and the coercive force (Hc) at the portion becomes small. Then, the
magnetization direction of the portion is arranged in the magnetization
direction of an external magnetic field, thereby recording information in
a form of recording bits.
When reproducing is to be carried out from the magneto-optical disk, a
linearly polarized light is projected onto the recording bits recorded on
the recording-reproduction film, and the information is reproduced
utilizing the Kerr effect (the rotation angle of the linearly polarized
light varies depending on the magnetization direction of the recording
bits). When adopting the above reproducing method, the recording bits
recorded with an interval smaller than the diameter of the light spot on
the recording-reproduction film cannot be reproduced.
In order to counteract the above problem, recently, a magneto-optical
recording disk which enables recording bits recorded with an interval
smaller than the diameter of the light spot has been proposed. Namely,
even when a plurality of recording bits are recorded in the area where the
light spot is formed, each of the recording bits can be reproduced (see
Jpn. J. Appl. Phys. Vol. 31 (1992) Pt. 1, No. 2B).
As shown in FIG. 27, the magneto-optical disk is mainly composed of a
substrate 81 whereon a read-out film 83 and a recording film 84 (magnetic
thin film with a perpendicular magnetization) are laminated. The recording
film 84 has high coercive force at room temperature. The coercive force of
the read-out film 83 is set smaller than the coercive force of the
recording film 84. When temperature of the reproducing portion of the
read-out film 83 is raised, the magnetization direction of the read-out
film 83 is arranged in the magnetization direction of the recording film
84. Namely, by the exchange coupling force exerted between the read-out
film 83 and the recording film 84, the magnetization of the recording film
84 is copied to the read-out film 83.
The recording on the magneto-optical disk is carried out through the
generally adopted magneto-optical recording method. When the recording
bits recorded on the magneto-optical disk are to be reproduced, the
read-out film 83 is required to be initialized beforehand so that the
magnetization direction of the read-out film 83 is arranged in a
predetermined direction (upward in the figure) by applying thereto the
subsidiary magnetic field from a subsidiary magnetic field generation
device 86. Then, the reproduction-use beam is projected onto the read-out
film 83 through a lens 98, and the temperature of the portion irradiated
with the beam is raised. As a result, the magnetized information recorded
on the recording film 84 is copied to the read-out film 83.
In this way, the magnetized information is copied only to the central
portion of the light spot of the reproduction-use light beam. As a result,
the recording bits recorded with an interval smaller than the light spot
can be reproduced.
However, in the above conventional arrangement, when reproducing is to be
carried out, the magnetization of the read-out film 83 must be initialized
beforehand by the subsidiary magnetic field generation device 86.
Therefore, the above arrangement presents the problem that the
magneto-optical recording and reproduction device becomes larger in size.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magneto-optical memory
device which permits high density recording by enabling recording bits,
recorded with an interval smaller than a diameter of a light spot, to be
reproduced and which permits the size of a device which adopts the
magneto-optical memory device and the electric power consumption to be
reduced.
In order to achieve the above object, the magneto-optical memory device in
accordance with the present invention is characterized by including:
a first magnetic film which exhibits in-plane magnetization at room
temperature and exhibits perpendicular magnetization in a predetermined
temperature range above room temperature;
a second magnetic film having Curie temperature set above room temperature;
and
a third magnetic film which exhibits perpendicular magnetization in a
temperature range between room temperature and Curie temperature, the
first magnetic film, the second magnetic film, and the third magnetic film
being laminated in this order on a base, and
wherein Curie temperature of the second magnetic film is set lower than the
Curie temperature of the third magnetic film.
In the above arrangement, when reproducing, a light spot is formed on the
first magnetic film at the portion irradiated with the light beam.
Additionally, the smallest limit of the size of the light spot is
determined by the wavelength of the light beam, the numerical aperture
(NA) of the objective.
When the light spot is formed on the first magnetic film, temperature of
the central portion of the light spot is raised in a predetermined
temperature range, and only the central portion of the light spot exhibits
perpendicular magnetization.
As a result, the recording bits recorded on the third magnetic film at high
density are copied to the portion which exhibits perpendicular
magnetization of the first magnetic film through the second magnetic film
by the exchange coupling force. Namely, the magnetization direction of the
third magnetic film is copied to the portion corresponding to the central
portion of the light spot on the first magnetic film.
Because only the central portion of the light spot on the first magnetic
film exhibits perpendicular magnetization, the recording bits recorded on
the third magnetic film with an interval smaller than the diameter of the
light spot can be reproduced by copying the recording bits recorded on the
third magnetic film to the first magnetic film, thereby enabling the
recording bits recorded at high density to be reproduced.
In the above arrangement, unlike the conventional model, the subsidiary
magnetic field generation device for initializing the magnetization of the
first magnetic film is not required when reproducing.
On the other hand, when recording is to be carried out, the temperature of
the portion which exhibits perpendicular magnetization of the third
magnetic film is raised to the vicinity of Curie temperature, and the
coercive force thereof becomes small. As a result, by changing the
magnetization direction of an external magnetic field, the magnetization
direction of the third magnetic film is recorded in a form of recording
bits.
In the above arrangement, since the second magnetic film, whose Curie
temperature is set lower than that of the third magnetic film, is provided
between the first magnetic film and the third magnetic film, the
temperature of the second magnetic film is raised to or above its Curie
temperature. Thus, the magnetization of the second magnetic film
disappears, and the exchange coupling force is not exerted between the
first magnetic film and the third magnetic film. Therefore, the
magnetization direction of the first magnetic film does not affect the
third magnetic film, thereby enabling the recording bits to be recorded on
the third magnetic layer by the external magnetic field smaller than that
required in the conventional model.
As described, because the above arrangement does not require the subsidiary
magnetic field generation device when reproducing, and the external
magnetic field required when recording can be made smaller, a compact size
for the apparatus and a reduction in electric energy consumption are made
possible.
For a fuller understanding of the nature and advantages of the invention,
reference should be made to the ensuing detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view showing a reproducing operation from a
magneto-optical recording medium adopted in the first embodiment of the
present invention.
FIG. 2 is an explanatory view showing the configuration of another
magneto-optical recording medium of the present invention.
FIG. 3 is a diagram showing the magnetic condition of a read-out film of
the present invention.
FIG. 4 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of room temperature--T.sub.1.
FIG. 5 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of T.sub.1 -T.sub.2.
FIG. 6 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of T.sub.2 -T.sub.3.
FIG. 7 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of T.sub.3 --Curie temperature.
FIG. 8 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of room temperature--T.sub.1 when a
material which makes the coercive force of the read-out film relatively
large is selected for the read-out film.
FIG. 9 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature of T.sub.1 -T.sub.2 when a material which
makes the coercive force of the read-out film relatively large is selected
for the read-out film.
FIG. 10 is an explanatory view showing the relationship between an external
magnetic field applied to the read-out film of FIG. 3 and a magnetic Kerr
rotation angle in a temperature range of T.sub.2 -Tc when a material which
makes the coercive force of the read-out film relatively large is selected
for the read-out film.
FIG. 11 is a longitudinal cross-sectional view showing a schematic
configuration of a magneto-optical memory device adopted in the second
embodiment of the present invention.
FIG. 12 is a longitudinal cross-sectional view showing a schematic
configuration of a magneto-optical disk as an example of the
magneto-optical memory device of FIG. 11.
FIG. 13 which shows a comparison example is a longitudinal cross-sectional
view showing the schematic configuration of the magneto-optical disk.
FIG. 14 is a longitudinal cross-sectional view showing the schematic
configuration of a sample used in measuring the respective characteristics
of the magnetic films a-f designed for the magnetic films of the
magneto-optical memory device of FIG. 11.
FIG. 15 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film a using the sample of FIG. 14.
FIG. 16 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film b using the sample of FIG. 14.
FIG. 17 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film c using the sample of FIG. 14.
FIG. 18 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film d using the sample of FIG. 14.
FIG. 19 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film e using the sample of FIG. 14.
FIG. 20 is graph showing the results of measured temperature dependency of
the coercive force of the magnetic film f using the sample of FIG. 14.
FIG. 21 is a graph showing the results of measured temperature dependency
of the Kerr loops of the magnetic films a-f using the sample of FIG. 14.
FIG. 22 is a graph showing the results of measured relationship between C/N
ratio and the size of the external magnetic field using a magneto-optical
disk A.
FIG. 23 is a graph showing the result of measured relationship between C/N
ratio and the size of the external magnetic field using a magneto-optical
disk B.
FIG. 24 is a graph showing the result of measured relationship between C/N
ratio and the size of the external magnetic field using a magneto-optical
disk C.
FIG. 25 is a graph showing the result of measured relationship between C/N
ratio and the size of the external magnetic field using a magneto-optical
disk D.
FIG. 26 is a graph showing the result of measured relationship between C/N
ratio and the size of the external magnetic field using a magneto-optical
disk E.
FIG. 27 is an explanatory view showing a reproducing operation from a
conventional magneto-optical disk which permits high density reproduction.
DESCRIPTION OF THE EMBODIMENTS
Embodiment 1
The following description will discuss the first embodiment of the present
invention with reference to FIGS. 1 through 10.
As shown in FIG. 1, a magneto-optical disk (magneto-optical memory device)
in accordance with the present embodiment is provided with a substrate 1
(base) whereon a transparent dielectric film 2, a read-out film 3 (first
magnetic film), a recording film 4 (third magnetic film), a transparent
dielectric film 5, and an overcoat film 9 are laminated in this order.
For the read-out film 3, amorphous alloy of rare-earth transition metal
(hereinafter referred to as RE-TM) is used. FIG. 3 shows the magnetic
condition of RE-TM. As can be seen from the figure, the range A where
RE-TM exhibits perpendicular magnetization at room temperature is
extremely narrow. This is because perpendicular magnetization appears only
in the vicinity of a compensating composition where the magnetic moments
of the rare-earth metal and the transition metal balance with one another.
Additionally, the magnetic moments of the rare-earth metal and the
transition metal have different temperature dependencies. Namely, when the
temperature is raised, the percentage of decrease in the magnetic moment
of the transition metal is less than that of the rare-earth metal.
Therefore, when the alloy (P in the figure) wherein the content of the
rare-earth metal is set greater than that in the compensating composition
at room temperature (Q in the figure) is adopted, the alloy can be
arranged such that it does not exhibit perpendicular magnetization but
exhibits in-plane magnetization at room temperature, whereas, it exhibits
perpendicular magnetization at above predetermined temperature.
More concretely, as the temperature of the portion irradiated with the
light beam is raised, the magnetic moment of the transition metal becomes
relatively greater until it balances with the magnetic moment of the
rear-earth metal, and the alloy exhibits perpendicular magnetization as a
whole. Therefore, by adopting alloy having the above characteristic for
the read-out film 3, the magneto-optical disk of the present embodiment
permits high density recording.
FIGS. 4 through 7 show relationship between the external magnetic field Hex
to be applied onto the read-out film 3 and the magnetic Kerr rotation
angle .theta.k (hysteresis characteristic). FIG. 4 shows hysteresis
characteristic in a temperature range of room temperature--T.sub.1. FIG. 5
shows hysteresis characteristic in the temperature range of T.sub.1
-T.sub.2. FIG. 6 shows hysteresis characteristic in the temperature range
of T.sub.2 -T.sub.3. FIG. 7 shows the hysteresis characteristic in the
temperature range of T.sub.3 -Tc.
As can be seen from the above figures, the alloy shows an abruptly rising
hysteresis characteristic in the temperature range of T.sub.1 -T.sub.3. On
the other hand, it does not show hysteresis characteristic in a
temperature range of room temperature--T.sub.1 and in a temperature range
of T.sub.3 -Tc.
In the present embodiment, Gd.sub.0.28 (Fe.sub.0.8 Co.sub.0.2).sub.0.72,
more preferably Gd.sub.0.28 (Fe.sub.0.82 Co.sub.0.18).sub.0.72, may be
adopted for the read-out film 3 with the thickness of 50 nm. Here, the
Curie temperature of the read-out film 3 is set in the range of 300-400
C.degree.. For the previously described reason, the read-out film 3 is set
such that the content of the rare-earth metal is set greater than that of
the compensating composition at room temperature, and that the
compensating composition appears at the vicinity of 100 C.degree..
For the recording film 4, Dy.sub.0.23 (Fe.sub.0.82 Co.sub.0.18).sub.0.77
with the thickness of 20 nm is adopted, and the Curie temperature is set
in the range of 150-250 C.degree..
For the transparent dielectric film 2, a dielectric film such as AlN, SiN,
AlSiN, etc., is used. Here, the thickness of the film is set substantially
the value obtained by dividing a quarter of a reproducing wavelength by a
refractive index. For example, when the light beam with the wavelength of
800 nm is adopted in reproducing, the film thickness of the transparent
dielectric film 2 is set in the range of 10-80 nm. Here, the transparent
dielectric film 5 is a protective coat made of nitride with the thickness
of 50 nm.
In the above arrangement, when reproducing operation is to be carried out,
a reproduction-use light beam 7 (perpendicular incident light) is
projected onto the read-out film 3 through a converging lens 8 from the
side of the substrate 1. As a result, the temperature of the portion of
the read-out film 3, corresponding to the vicinity of the central portion
of the light spot of the light beam 7 is raised, for example, to the
vicinity of 70.degree. C.
This is because the light intensity of the light beam 7 shows Gaussian
distribution, and temperature of the reproducing portion of the
magneto-optical disk also shows Gaussian distribution. Thus, the area
having a temperature rise above 70.degree. C. is smaller than the area of
the light spot of the light beam 7.
Therefore, in the arrangement of the present embodiment, for example, when
the information is recorded on the recording film 4 in the magnetization
direction shown in FIG. 1, in the area of the read-out film 3 having the
temperature rise above 70.degree. C., a transition occurs from in-plane
magnetization to perpendicular magnetization. As a result, by the exchange
coupling force exerted between the read-out film 3 and the recording film
4, the magnetization direction of the recording film 4 is copied to the
read-out film 3.
Further, when the transition occurs from in-plane magnetization to
perpendicular magnetization at the portion having a temperature rise of
the read-out film 3, only the portion corresponding to the central portion
of the light spot of the light beam 7 shows the magneto-optical Kerr
effect, and the information recorded on the recording film 4 is reproduced
based on the reflected light from the central portion of the light spot.
As described, because the reproducing operation is carried out only from
the portion having a temperature raise above 70.degree. C., the recording
bits recorded with an interval smaller than the diameter of the light spot
of the light beam 7 can be reproduced, and this permits a significant
improvement of the recording density.
On the other hand, temperature of the portion of the read-out film 3 other
than the portion corresponding to the central portion of the light spot of
the light beam 7 is not raised above predetermined temperature, and
in-place magnetization remains. Thus, the portion exhibiting in-plane
magnetization does not show the magneto-optical Kerr effect with respect
to the light beam 7.
Moreover, as the light beam 7 is shifted so as to reproduce the next
recording bit, the temperature of the previous reproducing portion drops,
and the transition occurs at the portion from perpendicular magnetization
to in-plane magnetization. With a drop in temperature, since the portion
does not show the magneto-optical Kerr effect anymore, entering of the
signal from the adjacent bits can be prevented, thereby preventing
generation of noise.
When recording and reproducing of information are carried out on and from
the above magneto-optical disk, only the portion corresponding to the
central portion of the light spot of the light beam 7 shows
magneto-optical effect. Thus, as long as the strength of the reproducing
signal is ensured, reproduction of the small recording bits is enabled,
for example, in the case where a plurality of recording bits exist in the
area where the light spot of the light beam 7 is formed, and the recording
density can be significantly improved. Moreover, the above arrangement is
improved from the conventional model in that the subsidiary magnetic field
generation device for initializing the magnetization of the read-out film
is not required.
In the above arrangement of the present embodiment, when the temperature of
the portion irradiated with the light beam 7 is raised above the vicinity
of 70.degree. C., the transition occurs from in-plane magnetization to
perpendicular magnetization. On the other hand, the temperature of the
other portion is not raised above the vicinity of 70.degree. C., and
in-plane magnetization remains. As a result, the reproduction of the
recording bits recorded with an interval smaller than the diameter of the
light beam 7 can be surely performed.
Additionally, the material for the read-out film 3 is not limited to
Gd.sub.0.28 (Fe.sub.0.8 Co.sub.0.2).sub.0.72. For example, Gd.sub.0.25
Co.sub.0.75 may be used as well. Since Gd.sub.0.25 Co.sub.0.75 has smaller
coercive force than Gd.sub.0.28 (Fe.sub.0.8 Co.sub.0.2).sub.0.72, when
recording, disturbing factor can be made smaller for the external magnetic
field.
Embodiment 2
The following description will discuss the second embodiment of the present
invention with reference to FIG. 2. A magneto-optical disk of the present
embodiment has the same arrangement as that of the first embodiment,
except that a reflecting film is provided.
As shown in FIG. 2, the magneto-optical disk (magneto-optical memory
device) of the present embodiment is provided with a substrate 11 whereon
a transparent dielectric film 12, a read-out film 13, a recording film 14,
a transparent dielectric film 15, a reflecting film 16, and an overcoat
film 19 are laminated in this order.
Although the reflecting film 16 is provided for enhancing the
magneto-optical effect, the substrate 11, the transparent dielectric film
12, the recording film 14, the transparent dielectric film 15, and the
overcoat film 19 of the present embodiment have the same configurations
and the characteristics as the substrate 1, the transparent dielectric
film 2, the recording film 4, the transparent dielectric film 5, and the
overcoat film 9 of the first embodiment. Thus, the detailed descriptions
thereof shall be omitted here.
In the present embodiment, the respective film thicknesses of the
transparent dielectric film 12, the read-out film 13, the recording film
14, the transparent dielectric film 15, and the reflecting film 16 are set
80 nm, 15 nm, 15 nm, 30 nm, and 50 nm.
In the arrangement of the present embodiment, a reproduction-use light beam
(not shown) is projected onto the read-out film 13 from the side of the
substrate 11 through the converging Kens (not shown). Among the components
of the light beam, those transmitted through the recording film 14 and the
transparent dielectric film 15 are reflected from the reflecting film 16.
In the above arrangement, when the information is recorded on the recording
film 14 in a predetermined magnetization direction (for example, in the
magnetization direction shown in FIG. 1), only the portion corresponding
to the central portion of the light spot of the light beam of the read-out
film 13 is raised to the vicinity of 70.degree. C. This is because the
temperature distribution of the read-out film 13 whereon the light beam is
projected shows Gaussian distribution.
As described, the transition occurs from in-plane magnetization to
perpendicular magnetization in the area having the temperature raise above
70.degree. C. Then, by the exchange coupling force exerted between the
read-out film 13 and the recording film 14, the magnetization direction of
the recording film 14 is copied to the read-out film 13. Based on the
light reflected from the area, the information recorded on the recording
film 14 is reproduced utilizing the magneto-optical Kerr effect.
In the present embodiment, since the reflecting film 16 is provided, the
magneto-optical effect is enhanced, and the magneto-optical Kerr rotation
angle becomes larger. As a result, the information can be more precisely
reproduced, thereby improving the quality of the reproducing signal in
addition to the effect of the first embodiment.
As described, in the portion other than the portion corresponding to the
central portion of the light beam, the temperature of the read-out film 13
is not raised, and in-plane magnetization remains. As a result, since the
magneto-optical effect is not shown with respect to the perpendicular
incident light beam, effect from the adjacent recording bits can be
avoided.
In addition, for the read-out film 13 is not limited to Gd.sub.0.28
(Fe.sub.0.8 Co.sub.0.2).sub.0.72, more preferably Gd.sub.0.28 (Fe.sub.0.82
Co.sub.0.18).sub.0.72 may be used. In replace of Gd.sub.0.28 (Fe.sub.0.8
Co.sub.0.2).sub.0.72, Gd.sub.0.25 Co.sub.0.75 may be used as well.
However, the materials for the read-out film 13 are not limited to the
above materials. The read-out film 13 does not necessarily show complete
in-plane magnetization as long as it shows substantially in-plane
magnetization. For example, Dy.sub.0.3 (Fe.sub.0.7 Co.sub.0.3).sub.0.7
which makes the coercive force relatively large may be used as well. When
adopting this material, the hysteresis characteristic thereof is as shown
in FIGS. 8 through 10.
FIG. 8 shows a relationship between the external applied magnetic field Hex
and the magnetic Kerr rotation angle .theta.k (hysteresis characteristic)
of the read-out film in the temperature range of room
temperature--T.sub.1. FIG. 9 shows a relationship between the external
magnetic field Hex and the magnetic Kerr rotation angle .theta.k
(hysteresis characteristic) of the read-out film in the temperature range
of T.sub.1 -T.sub.2. FIG. 10 shows a relationship between the external
magnetic field Hex and the magnetic Kerr rotation angle .theta.k
(hysteresis characteristic) of the read-out film in the temperature range
of T.sub.2 -Tc.
As can be seen from the above figures, when adopting Dy.sub.0.3 (Fe.sub.0.7
Co.sub.0.3).sub.0.7 which makes the coercive force relatively large, the
hysteresis characteristic is shown in the whole temperature range (room
temperature--Curie temperature Tc). Especially, an abruptly rising
hysteresis characteristic is shown in the temperature range of T.sub.1
-Tc. Here, T.sub.1, T.sub.2, and Tc represent the same temperatures as the
temperatures shown in FIG. 3.
Embodiment 3
The following description will discuss the third embodiment of the present
invention with reference to FIGS. 11 through 26.
As shown in FIG. 11, a magneto-optical disk (magneto-optical memory device)
in accordance with the present embodiment is provided with a substrate 21
(base) made of a transparent resin such as polycarbonate, whereon a
dielectric film 22, a magnetic film 23 (first magnetic film), a magnetic
film 24 (second magnetic film), a magnetic film 25 (third magnetic film),
a dielectric film 26, and a reflecting film 27 are laminated in this
order. The magnetic films 23-25 constitute the recording-reproduction
film.
For the magnetic film 23, the following material is used: The material
which shows in-plane magnetization at room temperature, and shows
perpendicular magnetization at above room temperature. For the magnetic
film 24, the magnetic material having Curie temperature set above room
temperature is used. For the magnetic film 25, the magnetic material which
shows perpendicular magnetization in the temperature range between room
temperature and Curie temperature is used.
The Curie temperature of the magnetic film 24 is set lower than that of the
magnetic film 25. Further, the magnetic film 23 is arranged such that as
the temperature thereof raises from the room temperature, transition
occurs from in-plane magnetization to perpendicular magnetization in the
temperature range between room temperature and the Curie temperature of
the magnetic film 24.
As examples, two kinds of magneto-optical disk A and B were prepared, and
three kinds of magneto-optical disks C, D, and E were prepared as
comparison examples.
As shown in FIG. 12, each of the magneto-optical disks A, B, D, and E is
provided with a substrate 31 (base) including a spiral-shaped pregroove
38, whereon a dielectric film 32, the magnetic film 33 (first magnetic
film), the magnetic film 34 (second magnetic film), the magnetic film 35
(third magnetic film), the dielectric film 36, and the reflecting film 37
are laminated in this order. Here, the magnetic films 33-35 constitute a
recording-reproduction film.
As shown in FIG. 13, the magneto-optical disk C is provided with the
substrate 31 including a spiral-shaped pregroove 38, whereon the
dielectric film 32, the magnetic film 33, the magnetic film 35, the
dielectric film 36, and the reflecting film 37 are laminated in this
order. Here, the magnetic films 33 and 35 constitute a
recording-reproduction film.
A glass is used for the substrate 31. As to the material for the dielectric
film 32, AlN with the thickness of 80 nm is used. For the magnetic film
33, a magnetic film a with the thickness of 40 nm (see Table 2) such as
Gd.sub.0.26 (Fe.sub.0.80 Co.sub.0.20).sub.0.74 is used. For the magnetic
film 34, either one of the magnetic films c, b, e, and f (see Table 2)
with the thickness of 10 nm is used. As to the material for the magnetic
film 35, the magnetic film d with the thickness of 40 nm, such as
Dy.sub.0.23 (Fe.sub.0.80 Co.sub.0.20).sub.0.77 is used. For the dielectric
film 36, AlN with the thickness of 20 nm is used. For the reflecting film
37, Al with the thickness of 30 nm is used.
The magnetic film a made of GdFeCo is RE-rich at room temperature. The
compensation temperature of the magnetic film a is 270.degree. C., and the
Curie temperature thereof is set at 380.degree. C. The magnetic film b
made of GdFeCo is RE-rich in the temperature range between room
temperature and Curie temperature (160.degree. C.). The magnetic film c
made of GdFeCo is RE-rich in the temperature range between room
temperature and Curie temperature (250.degree. C.).
On the other hand, the magnetic film d made of DyFeCo is TM-rich in the
temperature range between room temperature and Curie temperature
(230.degree. C.). The magnetic film e made of DyFeCo is TM-rich in the
temperature range between room temperature and Curie temperature
(160.degree. C.). The magnetic film f made of DyFeCo is TM-rich in the
temperature range between room temperature and Curie temperature
(250.degree. C.).
Here, RE-rich at room temperature indicates that the content of RE
(rare-earth metal) is greater than the content of RE when the compensation
temperature is set at room temperature. RE-rich from room temperature to
Curie temperature indicates that the content of RE is greater than the
maximum content of RE when the compensation temperature is set in the
range between room temperature and Curie temperature. TM-rich from room
temperature to Curie temperature indicates that the content of TM
(transition metal) is greater than the maximum content of TM when the
compensation temperature is set in the range between room temperature and
Curie temperature.
The magnetic film a is used as a read-out film. The compensation
temperature thereof is set higher than the compensation temperature (in
the vicinity of 100.degree. C.) of the read-out film 3 used in the first
embodiment in the following reason: The magnetic film a is arranged so as
to exhibit perpendicular magnetization in the temperature range of
130.degree.-280.degree. C., and reproduction is carried out from the
portion corresponding to the central portion of the light spot of the
light beam, having the temperature above 130.degree. C.
This means that the area of the reproducing portion which exhibits
perpendicular magnetization is smaller than the reproducing portion (area
having temperature rise above 70.degree. C.) of the first embodiment.
Since the arrangement of the third embodiment enables the recording bits
smaller than the first embodiment to be reproduced, it is more suitable
for the high density reproduction.
As shown in FIG. 14, the magnetic characteristic is measured using the
sample provided with the substrate 41 whereon an AlN film 42, a magnetic
film 43 corresponding to either one of the magnetic films a-f, and the AlN
film 44 (coating film) are laminated in this order. FIGS. 15 through 20
show respective temperature dependencies of the coercive forces (Hc) of
the magnetic films a-f. The coercive force of the third magnetic layer is
always stronger than the coercive force of the second magnetic layer in a
temperature range of from not less than room temperature to less than the
Curie temperature (T.sub.3) of the third magnetic layer. FIG. 21 shows
temperature dependencies of Kerr loops of the magnetic films a-f
respectively. Here, the Kerr loop indicates hysteresis characteristic of
the Kerr rotation angle with respect to the change in the external
magnetic field.
Table 1 shows the magnetic films a-f used for the magnetic films 33-35 of
the magneto-optical disks A-E. Table 2 shows respective compositions and
magnetic characteristics of the magnetic films a-f. In Table 2, the
transition indicates that the magnetization direction is in-plane
magnetization at room temperature, and it is perpendicular magnetization
in a predetermined temperature range above room temperature.
TABLE 1
______________________________________
A B C D E
______________________________________
magnetic film 33
a a a a a
magnetic film 34
b e -- c f
magnetic film 35
d d d d d
______________________________________
Using the above magneto-optical disks A-E, the recording bits with the size
of 0.5 .mu.m were recorded while modulating the size of the external
magnetic field. Then, the recording bits were reproduced, and the C/N
ratio (carrier/noise ratio) was measured. In the experiment, linear
velocity of the magneto-optical disks A-E, the laser power when recording,
and the laser power when reproducing were set respectively 5 m/s, 8 mW,
and 2 mW. The results of the experiment using the magneto-optical disks
A-E are shown in FIGS. 22 through 26.
As can be seen from FIGS. 22 and 23, as to the magneto-optical disks A and
B, when the size of the external magnetic field was set above 200 Oe, the
C/N ratio above 45 dB was measured. On the other hand, as to the
magneto-optical disks C, D and E, the C/N ratio above 45 dB was not
measured until the external magnetic field was set above 1000 Oe as shown
in FIGS. 24 and 26.
As described, each of the magneto-optical disks A and B is provided with
the magnetic film 34 between the magnetic film 33 and the magnetic film
35, and the Curie temperature of the magnetic film 34 is set lower than
that of the magnetic film 35. On the other hand, the magneto-optical disk
C is not provided with the magnetic film 34. As to the magneto-optical
disks D and E, although each of which is provided with the magnetic film
34, the Curie temperature of the magnetic film 34 is set higher than the
Curie temperature of the magnetic film 35. When the magneto-optical disks
A and B were used, the recording was permitted with a small external
magnetic field. However, when the magneto-optical disks C, D, and E were
used, the recording was permitted only with a large external magnetic
field.
From the result of the experiment using the magneto-optical disks A and B,
both the perpendicular magnetization film and the in-plane magnetization
film may be used for the magnetic film 34.
TABLE 2
__________________________________________________________________________
film (nm)
Hc (kOe)
Tcomp
Tc magnet.
composition thick.
at Troom
(C..degree.)
(C..degree.)
direction
__________________________________________________________________________
a Gd.sub.0.26 (Fe.sub.0.80 Co.sub.0.20).sub.0.74
40 0.03 270 380 transition
b Gd.sub.0.28 (Fe.sub.0.90 Co.sub.0.10).sub.0.72
10 0.02 -- 160 in-plane
c Gd.sub.0.28 (Fe.sub.0.85 Co.sub.0.15).sub.0.72
10 0.02 -- 250 in-plane
d Dy.sub.0.23 (Fe.sub.0.80 Co.sub.0.20).sub.0.77
40 13 -- 230 perpend.
e Dy.sub.0.22 (Fe.sub.0.90 Co.sub.0.10).sub.0.78
10 13 -- 160 perpend.
f Dy.sub.0.24 (Fe.sub.0.77 Co.sub.0.23).sub.0.76
10 14 -- 250 perpend.
__________________________________________________________________________
As described, the magneto-optical memory devices A and B of the present
embodiment permits a reduction in the strength of the external magnetic
field required when recording. This can be achieved in the following
reason: The portion of the magnetic film 34 whose temperature is raised
above its Curie temperature, the exchange coupling force is not exerted
between the magnetic film 33 and the magnetic film 35. Therefore, the
effect from the magnetization of the magnetic film 33 can be avoided. As a
result, the recording onto the magnetic film 35 can be more easily carried
out.
As described, the magneto-optical disk of the present embodiment is
arranged so as to be provided with the substrate 31 whereon the magnetic
film 33 which exhibits in-plane magnetization at room temperature, and
exhibits perpendicular magnetization in a predetermined temperature range
above room temperature, the magnetic film 34 whose Curie temperature is
set above room temperature, and the magnetic film 35 which exhibits
perpendicular magnetization in the temperature range between room
temperature and the Curie temperature are laminated in this order.
In the above arrangement, when reproducing, the information recorded on the
magnetic film 35 at high density can be reproduced by the exchange
coupling force exerted among the magnetic films 33, 34, and 35. Namely,
the recording bits recorded with an interval smaller than the diameter of
the light spot can be reproduced.
In the conventional model, in order to reproduce the recording bits
recorded with an interval smaller than the diameter of the light spot,
subsidiary magnetic field for initializing the magnetic film (read-out
film) is required, and for generating the subsidiary magnetic field, a
magnetic field generation device is provided.
However, in the arrangements of the above embodiments, the magnetic field
generation device for generating the subsidiary magnetic field is not
required. Thus, the apparatus can be made smaller, and the electric power
consumption can be reduced.
Moreover, when recording, since the magnetic film 34 whose Curie
temperature is set lower than the Curie temperature of the magnetic film
35 is provided between the magnetic films 33 and 35, the temperature of
the magnetic film 35 can be raised above the vicinity of its Curie
temperature. Thus, the temperature of the magnetic film 34 can be also
raised above its Curie temperature, and the magnetization disappears from
the magnetic film 34. As a result, the exchange coupling force is not
exerted between the magnetic films 33 and 35.
By providing the magnetic film 34, the magnetic film 35 can be avoided from
being affected by the magnetic film 33. The magnetic film 34 also serves
to prevent the external magnetic field by leakage from the magnetic film
35. This permits the information of the recording bits to be recorded onto
the magnetic film 35 using the external magnetic field smaller than that
required in the conventional model.
In the above arrangement, in the case where the external magnetic field is
generated using the electro-magnet, electric power consumption when
recording can be reduced. When a magnet is used for generating the
external magnetic field, by adopting the magnet smaller that required in
the conventional model, the apparatus can be made more compact.
In the described embodiments 1-3, the magneto-optical disk has been used as
an example of the magneto-optical memory device. However, the present
invention is not limited to this. For example, a magneto-optical card or a
magneto-optical tape may be used as well. When adopting the
magneto-optical tape, in stead of the substrate 31, a tape base (base)
such as polyethylene terephthalate may be used as well.
The invention being thus described, it will be obvious that the same way be
varied in many ways. Such variations are not to be regarded as a departure
from the spirit and scope of the invention, and all such modifications as
would be obvious to one skilled in the art are intended to be included
within the scope of the following claims.
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